1. Field of the Invention
This invention relates generally to a system and method for detecting a phase change from liquid to gas and, more particularly, to a system and method for detecting a phase change from liquid to gas through an anode recirculation bleed/drain valve so that an anode bleed model knows the amount of nitrogen that is being bled from the anode side of a fuel cell stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is desirable that the distribution of hydrogen within the anode flow channels in the fuel cell stack be substantially constant for proper fuel cell stack operation. Therefore, it is known in the art to input more hydrogen into the fuel cell stack than is necessary for a certain output load of the stack so that the anode gas is evenly distributed. However, because of this requirement, the amount of hydrogen in the anode exhaust gas is significant, and would lead to low system efficiency if that hydrogen were discarded. Further, hydrogen gas in a sufficient quantity discharged to the environment could cause certain problems because of the combustible nature of hydrogen. Therefore, it is known in the art to recirculate the anode exhaust gas back to the anode input to reuse the discharged hydrogen.
The MEAs are porous and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve at the anode gas output of the fuel cell stack to remove nitrogen from the anode side of the stack.
Models are typically used to calculate the concentration of nitrogen in the anode side of the fuel cell stack based on the operating parameters of the fuel cell system, such as stack current density, system pressure, etc.
As mentioned above, water is a by-product of the fuel cell stack operation. Water is forced out of the anode flow channels by gas flowing therethrough. The water expelled from a fuel cell stack is typically collected in a holding tank in a water separating device in the anode exhaust flow system. A water level indicator provided in the water separator device indicates when the tank is full and a drain valve is subsequently open to drain the tank to the environment.
It has recently been proposed in the art to reduce the complexity of a fuel cell system by combining an anode bleed valve and an anode drain valve into a single valve to perform both the bleed and drain functions discussed above. This combined drain and bleed valve has been proposed to be located in the water separation device at the bottom of the holding tank. However, when a bleed is commanded to remove nitrogen from the anode side of the stack, water in the holding tank must first be removed before gas in the anode exhaust can flow through the valve in the water separation device. In order for the model that determines the amount of nitrogen in the anode side of the fuel cell stack to be accurate, it needs to know the phase transition from when the bleed/drain valve is draining water to when it is bleeding gas so that the model knows that nitrogen is being removed from the anode side of the stack.